When you are grinding with a girl, or doing any chest-to-back dance, and you get an erection that is therefore "scratching" her back. Depending on the severity of it and how intoxicated the girl is she may not feel it at all.

Ex. I popped 2 Viagra before coming to the club, girls get ready for the back scratcher.

A racial term for an illegal Mexican immigrant. It originated from the myth that Mexicans crawl under the U.S fence to get into America. In the proccess, their backs get scratched from the fence. (Also, see "wet backer")